Session 2 Populations, Variation, and Evolution

Natural Selection & Evolution – POPULATIONS EVOLVE NOT INDIVIDUALS. This is because we “are” what we “are” because your genes. You can’t change your somatic cells’ DNA by choice, only by random mutation. If a mutation occurs in the DNA in the gametes (sperm and eggs), then the NEXT generation of offspring has a change in traits The “population” is evolving. “Evolve” just means “change over time” and that is what has occurred.

You are what you inherit in terms of DNA. The egg surrounded by sperm.

Watch Video Homer Simpson Evolution Teaching Science at blinkx.mp4 Watch Video Homer Simpson Evolution Teaching Science at blinkx.mp4

– Individuals “suffer” or “benefit” as a result of the traits they inherited or mutations they acquire during their life. “Weak” vs. “strong” genes is the way it is usually communicated. “Weak” are considered detrimental traits and “strong” are considered favorable traits in terms of survival and reproduction.

Population

Population Genetics – The science that studies the trait variation rates over time within a population. – It basically is following allele frequency rates in a gene pool. (A.K.A. a population.)

Population is defined by four criteria: – SAME species of organism. – Located in the SAME location. – At the SAME time. – And showing signs of reproduction. (Offspring are present within the group.)

Species Male and Female Blue Footed Boobies

Species – Mostly defined as organisms that are so genetically similar in genome that there exists the potential to breed and produce viable (living) fertile (able to reproduce eventually themselves) offspring. – Genetics are very similar is important to defining a species since it is the “blueprint” for “constructing” an organism. The “plans” must be very similar or there will be confusion in “construction” and problems will arise during development. Problems are a terrible thing to encounter since we are discussing the making of a LIVING organism.

Geographic range vs. population A population is in one SPECIFIC given area; but in the case of organisms that are quite common (For example, grey squirrels or humans.); we may have several populations that cover a wider expanse of territory. In the case of humans, as a species we are global in our range; but we have millions of different populations, such as the population of Montgomery. “Range” refers to EVERYWHERE where that species MAY be found. Gene pools may or may not interact; it depends on the species and if any geographic barriers (such as large mountains or large bodies of water) interfere with the ability to interact.

Geographic Range of west coast salamanders

Allele frequency (Remember, an allele is a version of a gene.) – “Frequency” refers to “how many” are present at that time within the population (gene pool). – Is considered fixed, if there is NO change in frequency—NO evolution is present. (A state of equilibrium is occurring.) – Is considered evolving, if frequency is changing— evolution is occurring. (A state of change is occurring over time.)

Geographic Range of Humans

Variation (Different traits within a species or population.) Variation is key to surviving in a changing environment. (OPTIONS) Some members will survive and reproduce. These OPTIONS are the raw building materials of evolution to utilize. Variation, will only come into existence with a change in the DNA nucleotide sequence, ie. mutation. Some are favorable, MOST are harmful.

Variation Exists Variation exists between individuals and populations.

Variation Exists

“Creating” Variation for evolution to build upon: A. Through mutations These changes are rare and random in gametes. (Because these cells are normally not exposed to environmental stresses an organism may encounter in their existence.) Mutations mostly occur in somatic cells because these cells are exposed to the environmental stresses. Most mutations, are harmful to the cell or organism, so it usually dies.

Variation in Crossover

Through sexual reproduction – The process of crossover, during Prophase I of meiosis, “swaps genes from one chromosome to another, its equal “mate” usually, during gamete formation. (This is so that each sperm or egg is unique in it’s genetic composition.)

Variation in Anaphase I

The Segregation (means “separation”) of Chromosomes during Anaphase I and II of meiosis so as to reduce the genetic content (number of chromosomes) found within a sperm or egg to one- half (referred to as “haploid”) the normal content (referred to as “diploid”).

Variation in Anaphase II

The random fertilization of a sperm by an egg increases variety. (Each sperm and egg are different remember… so each coming together between egg and sperm (what is referred to as fertilization) will be different too.)

Variation in Egg and sperm (Which sperm will it be?)

Phenotypic Polymorphism (A.K.A. Discrete Characters.) A. These are referred to as single gene traits for the “discrete” phenotypic outcome. B. They are also called either – or – traits (You either have the gene or you don’t, which in turn means you either produce the trait or you don’t.)

Phenotypic Polymorphisms Witches Peak (forehead)

C. Phenotypic (means “the physical outcome of a gene); polymorphisms (means “many versions or types”) 1. This basically means that there may exist multiple versions of the same basic gene trait simply because of a different sequence of nucleotides that make that gene. A, B, AB, and O Blood types for example. (These are all blood; but different versions or types of the same main gene being expressed.)

Phenotypic Polymorphisms Blood Types

Genotypic Polymorphisms (A.K.A. Quantitative Characters.) – These are traits for which there may exist several phenotypic outcomes based on the fact that these traits are the cumulative interaction of several genes interacting with one another. – The fact that there are several genes involved is why they are also referred to as Quantitative (how many alleles) characters.

Genotypic Polymorphisms Skin pigmentation “Coloration”

Skin color is basically based upon how many dominant alleles you inherit from your parents. The dominant allele makes the protein pigment found in skin called melanin. Every human makes this protein pigment, except Albinos (a genetic condition with ZERO dominant alleles and do not have a single DNA blueprint for making the protein pigment; so they “appear” white because they have zero pigment in their cells. So basically, the more dominant alleles that are inherited; the darker the skin will be. Race is a man made construct that groups individuals based on geography or degree of pigmentation, BUT WE ARE ALL HUMAN. We now recognize the direct relationship between the environment and the degree of pigmentation. Populations around the Equator are darker (to counteract the harmful effects of the sunlight) and away from the equator the amount of pigmentation decreases.

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Genetic Drift causing CHANGE in allele frequency of a population. (“drift” indicates “random”) A. Random (unpredictable) change in allele frequency that occurs within a population’s gene pool. This process is more severe on small parent populations rather than large populations usually.

Genetic Drift “Random change”

LE 23-7 CRCRCRCR CRCRCRCR CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CWCWCWCW CWCWCWCW CRCWCRCW CRCWCRCW CRCRCRCR CRCWCRCW CRCWCRCW CRCRCRCR CRCRCRCR CRCWCRCW CWCWCWCW CRCWCRCW CRCRCRCR Only 5 of 10 plants leave offspring Only 2 of 10 plants leave offspring CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3

Two ways that random drift can occur: – Bottleneck effect (This has a negative effect by reducing variation.) This is usually caused by a random act of nature, such as a hurricane. Imagine an island with a parent population consisting of variation within the gene pool. A hurricane ravishes the island and most of the individuals of that species die. The survivors (the individuals who made it out of the bottle’s neck) are now the sole survivors. They comprise the NEW gene pool for that population and the allele frequency HAS dramatically changed because of this unpredictable event.

LE 23-8 Original population Bottlenecking event Surviving population

Founder effect (This also has a negative effect by reducing variation.) This is also caused by a random event. In this situation, a small part of a parent population leaves or becomes isolated from the larger parent population. This leaving or isolation “creates” a new Founding gene pool for the area they will occupy.

“Founding fathers”

Think of the “founding fathers” leaving Europe or Africa for America. They will “create” a NEW gene pool (with a NEW allele frequency) for their NEW environment. The larger older parent population MAY have its allele frequency changed as well.

“Founding fathers”

Natural Selection – ALWAYS has a positive effect on variation because “nature” favors those traits that make a population or species more able to survive within an environment and increases their ability to reproduce and keep the species viable. The weak traits perish over time.

Gene Flow (Movement on purpose)

Gene Flow (Flow indicates purposeful movement between populations.) A. This may have a negative effect, a positive effect, or no effect on allele frequency within a population. B. Migration in and out of an area MAY affect the gene pool. C. Reduced variation can occur if the interaction between populations is permitted to occur. They may begin to merge into one unified population instead of two distinct populations. Variation may also increase if new “traits” are introduced to a stagnate gene pool. Usually seen in captive breeding of endangered species.

Gene Flow (Movement on purpose)

Evolutionary flow (Modes or ways of selection “affecting” phenotypic outcomes.) A. Directional – the “bell curve” for a trait flows in ONE DIRECTION ONLY. B. Diversifying (Disruptive) - the “bell curve” for a trait separates in opposite directions at the SAME time. C. Stabilizing – the “bell curve” moves to the “stable” center. D. These are ALL related to some trait that is beneficial for survival within that changing environment.

Evolutionary Flow

Hardy-Weinberg Theorem – This SET of equations is used to follow allele frequency within a population (also considered a gene pool) If the numbers (rates) CHANGE from generation to generation, the population is evolving over time. If the numbers (rates) DO NOT CHANGE from generation to generation, the population is not evolving over time and is then said to be in a state of equilibrium. – Equation #1: p + q = 1 (This equation is for alleles.)(“p” refers to the “dominant” allele percentage and “q” refers to the recessive allele percentage.) Together p + q percentages must equal 100% of the gene pool or 1.

Hardy – Weinberg Theorem Equation 1 (p + q= 1)

– Equation #2: p 2 + 2pq + q 2 = 1(This equation refers to the percent composition/number of organisms within the population (gene pool) at that time.(It is essentially a Punnett square, BUT in math format.) p 2 = the homozygous dominant percentage of organisms within the population at that time. 2pg = the heterozygous percentage of organisms within the population at that time. q 2 = the homozygous recessive percentage of organisms within the population at that time. ALL THREE MUST ADD UP TO 100% (1) of the population.

Fig: 23.5 Hardy – Weinberg Theorem Equation 2 (p² + 2pq + q² = 1)

– These equations are mainly used in health sciences to explain the frequency of genetic conditions. – These equations can be used to show how or if variation is preserved over time. – Five conditions must be met for a population to be in Equilibrium (Frequency NOT changing):

The five conditions for non-evolving populations are rarely met in nature: – Extremely large population size – No gene flow – No mutations – Random mating – No natural selection